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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW

By G8MNY (Updated Mar 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

This is a simple AF 5W booster amp for handhelds etc. I built mine into a large RF PA box & included a 5" LS so it is really loud. It has the advantage of being fairly RF immune unlike IC Amplifiers. Handhelds & small rigs don't produce a full power rail LS swing (often on low voltage) so a small 2:1 step up transformer is used to make sure the double emitter follower current gain circuit can go into full clipping with low drive swing. Using a low 4Ω or 3Ω LS plenty of AF power is available.

┌────────┬────────────────┬─────o──o─<+12V
C1 47uF 1k │ │+ 3A
Rig LS>─┬──┤├───┬───┐ 220uF │ TR2│/ C6 === 470uF
│ + )║(∙ ┌───┤├────┴──┬───┤ NPN │ 18V
│ 2x )║( │ C3 + _│_ │\e + 1000uF │
│ Step )║( │ D1 \_/ ├────┤├──┐ 10V │
100Ω up ∙)║(___│ C4 + │ │/e C5 │ │
│ T1 │ │ └───┤├────┬──┴───┤ PNP ┌┴┐/│4Ω │
│ │ === 220uF │ TR1│\ └┬┘\│LS │
│ │ C2│SOT 10V 1k Ge │ │ 8W │
─────┴───────┴────┴───────────┴────────┴────────┴───────┴─────────-< -ve

INPUT CIRCUIT
The input 100Ω is only there to provide a DC path if the rig needs a DC load for bootstrap current etc. It can be omitted to save handheld battery life if this is not required.

The input load on the handheld will be the 1/4x (LS x HFE // 500Ω) which should be no lower than 15Ω.

T1 is typically a small transistor radio output transformer of 50mW or so rating, that has 2 identical windings. This ensures the Transistors will be driven with voltage swings above & below the power rails for max power.
_
+4V_ _ +12V~│ / \ /~\
│/ \ ││ │ │ │
_│ \_/ │ │ │ │ │
-4V 50mW 0V_│ \ / \_/
Input Base ~ Drive Full Swing
> 0 & 12V Output 5W

The value of C1 limits the LF response, & with the inductance of T1 plus the input load can give a good LF cut off below 300Hz of a rate of 12dB/Octave.
C1 only needs to be low voltage type.

6dB/O LF cut 6dB/O HF cut
─┤├──┬───>to LOAD ──((()──┬──>to Load
C1 )║ ==== │
L )║ Another L === Another
)║ 6dB/O C2 │ 6dB/O
─────┴─── ────────┴───

Gain
0┐ .──────────────.
-3dB┤ / \
│ / \
-12dB┤ / \
├───┬───┬───┬────┬────┬───┬───┬───┐ Hz
0 80 150 300 700 1k5 3k 6k 12k

The value of C2 is best found on test (SOT), it's effect together with the non mutual inductance leakage L of T1 provide a sharp HF cut off point above 3kHz & also gives a slope of 12dB/Octave. Typical C2 Value is 1nF.

BIASING
A single Silicon diode D1 is used with 2x 1k to give the required 0.7V bias @ half power rail point. To maintain the bias over the input current drive cycle 2 large capacitors C3 & 4 are used. These only need to be 10V working.

TRANSISTORS
Use a metal box for the construction & bolt down TR1, a PNP Germanium transistor like a TO3 OC35, with no insulation kit, & TR2 a Silicon NPN like a 2N3055 with an insulation kit. As these are a double emitter follower output, then the bias voltage of 0.7V is just about right, so no emitter Rs are needed.

As there is 100% NFB the distortion is low even with non gain matched devices, but there will be some low level crossover distortion!

OUTPUT
This is fed through a large C5 1000uF to maintain the low output Z & low losses. Unlike IC amplifiers were the peak LS voltage is typically ±1V less than the power rails, in this design it will be very close to full power rails. So quite a bit more power is available. e.g. on 14V it should give ±6.8V swing into 4Ω giving about 5.8W RMS 11.6W "USA music rating" (e.g. full square wave power.)

For stability C6 is included across the power rails.


Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP

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